This invention relates to control systems for machines and more particularly to control systems for cyclic machines (i.e. for machines which are operable in cycles).
Typically, cyclic machines carry out a number of functions during each cycle, each of which must start at a predetermined stage of the machine in its cycle (which stage may or may not be unique for each function) and stop at a second predetermined stage in the machine cycle (which again may or may not be unique for each function). These functions often are interrelated, and thus it is important that they start and stop as nearly as possible at the predetermined stages in the machine cycle. Cycle rates of 100, 200 or even 300 cycles/minute are not uncommon. At these rates, accurate control of several functions during each cycle is difficult.
For many years, control systems have used rotary cam actuated limit switches for controlling machine functions. But this requires that a fairly large number of cams be accurately positioned to strike the limit switches at precise stages in the machine cycle. Adjustment of the cams and switches is difficult. And failure after prolonged use is common because of mechanical wear and electrical deterioration of the switches. Particularly at higher cycle rates, cam based systems are unsatisfactory Solid state control systems have been developed to address some of these problems, but present ones also have disadvantages. Foremost among these disadvantages is cost. Solid state control systems are relatively expensive to purchase. In addition some of these systems require additional programming before they can be used with a particular type of machine, which can also be expensive.
A second disadvantage of solid state control systems is that some are too complex for many machines, particularly for machines having only a relatively small number of functions and requiring only limited logic capability. For example, the packaging industry uses form, fill and seal machines which have only a few (e.g., six) functions to be controlled. Such functions might include the feeding of flexible packaging material to the machine, pulling or feeding the packaging material over a mandrel to form tubing and past a sealing station, opening and closing sealing bars or dies at the sealing station to form packages, cutting the packages apart, signalling a product feeding apparatus to feed more product to be packaged, etc. Such a machine is shown in coassigned U.S. Pat No. 4,288,965. These functions can be controlled by timers and in the case of a vertical form, fill and seal machine nine to twelve timers would be sufficient. Doing this causes an additional problem, however, whenever the speed of the machine is changed. When this happens all the timers must be readjusted In fact, machine speed is usually not constant but instead varies over some small range which means that the timing of the functions will never be exactly right even at a nominally constant speed. This problem can be solved by making the machine speed truly constant, but this is expensive.
Previous form, fill and seal machines inhibited the operation of various functions to ensure that the functions occurred in proper order with respect to each other and with respect to the feeding of product to the machine. Concerning the latter, product feed is often an asynchronous operation with respect to the packaging machine As a result, previous machines sometimes have had to skip a cycle (which is called a "dry cycle") because the product at that particular time is not being supplied to the packaging machine at a high enough rate.
Another group of problems with previous form, fill and seal machines occurs when packaging material having registration marks is used to make packages Generally, these registration marks (call "eyespots") are spaced at package length intervals along the material. If for some reason one of these registration marks is not detected by the machine, the machine quickly gets out of registration with the material at the proper point, i.e., within a predefined window. This is a problem with printed packaging material. If the out of registration condition continues for several cycles, total loss of registration can result.
Another problem with printed packaging material is that the machine may mistake some of the printing for an eyespot, which again causes registration to be lost. A third problem, peculiar to a certain type of packaging machine, namely those having a "fixed eye" for detecting the registration marks, concerns synchronizing the sealing of a package with the detection of an eyespot. With a movable detector, the detector can simply be moved by the operator during setup of the machine for a particular run of material until the sealing occurs at the proper place. With a machine having a "fixed eye", this is not always possible Although "fixed eyes" can often be moved a few inches along the path of the packaging material, this is often not enough to permit sealing of the packages at the required spot. One possible solution to this problem is to provide a preset time delay between the detection of an eyespot and the sealing of the package Doing this causes yet another problem, viz. a change in machine speed will cause the length of material fed past the sealing station during the preset time delay to vary, causing loss of registration.